The 3rd Generation Partnership Project, 3GPP, is responsible for the standardization of the Universal Mobile Telecommunication System, UMTS, and Long Term Evolution, LTE. LTE is also sometimes referred to as Evolved Universal Terrestrial Access Network, E-UTRAN. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is a next generation mobile communication system relative to UMTS. LTE brings significant improvements in capacity and performance over previous radio access technologies.
The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment, UE, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNB or eNodeB, in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
The ever increasing end-user demands are a significant challenge to the operators. Separating users spatially by means of precoding and/or beamforming is one way to improve the performance of a wireless system. Such techniques include Multi-User Multiple-Input Multiple-Output, MU-MIMO, coordinated beamforming and multi-user coherent joint transmission, where the two latter belong to the family of Coordinated Multi Point, CoMP, methods.
These techniques all use coherent transmission from many transmitter antennas that are not necessarily in the same physical site to multiple receiver antennas also not necessarily in the same physical location or UE, to extend classical MIMO spatial multiplexing to the Multi-User case. The coherent transmission is typically not only aiming at maximizing the received signal power at the desired receiver, but also at reducing interference to all non-desired receivers. In some cases transmission, TX, antennas are located in an antenna grid where all antenna elements effectively help to create a narrow beam towards the desired receiver. This is essentially the same as classical beamforming, but typically done considering several users at the same time. Hence, beams are directed to several users. In other cases, the TX antennas are spread out over the physical network, similar to today's cellular network deployments, but with a coherent transmission instead of today's uncoordinated each-cell-for-itself transmission.
Hence, the Coordinated Multipoint in LTE is essentially a range of different techniques that enable the dynamic coordination of transmission and reception over a variety of different base stations. The aim is to improve overall quality for the user as well as improving the utilization of the network. In CoMP the eNB:s dynamically coordinate their transmissions to provide joint scheduling and transmissions, as well as proving joint processing of the received signals. In this way a UE at the edge of a cell is able to be served by two or more eNBs to improve signals reception/transmission and increase throughput particularly under cell edge conditions.
When using coordinated multipoint transmission, clusters are defined in order to be able to coordinate the transmissions of different transmission antennas. A CoMP cluster is a set of transmission antennas which are transmitting in a coherent and coordinated way to a set of wireless devices. There are multiple CoMP clusters in a network. Coordination between different CoMP clusters is often not practically possible. However, in any practical Multi-User Joint Transmission deployment, the size of each cluster will have to be limited in order to create a reasonable load on the network baseband, backhaul communication etc. This will typically lead to incoherent interference between clusters. Though, today's precoder algorithms typically do not take this inter-cluster interference into account.
Hence, there is a need for improved precoder algorithms to be used for coordinated multipoint transmission applications.